Field of the Invention
The invention relates in general to surface metrology. In particular, it relates to a combination of interferometric and so-called depth-from-focus data processing with side illumination added to conventional scanning interferometers.
Description of the Related Art
Interferometric techniques for surface profilometry (such as vertical-scanning and phase-shift interferometric methods) are well known in the art. They are particularly suitable for measuring smooth surfaces with relatively low slopes. When a high slope is encountered, interferometric fringes are weak or are not produced at all because the light is scattered away from the objective and no useful data are acquired. This problem has been addressed in scanning interferometry by rotating the sample between measurements to ensure that the slope of the surface being measured is such that enough light is returned to the objective to produce fringes at all times; the measurements are then stitched together in some manner to yield the complete profile of the surface. This approach requires very precise sample manipulation and data processing is complicated by the need to align each set of measurements for stitching purposes.
Another approach has been to provide an instrument with multiple-level illumination capability in order to increase the irradiance to the sample when higher slopes are measured (and vice-versa when the slope decreases) or when low-reflective samples are measured. See, for example, J. Schmit and A. Harasaki, “High precision shape measurement of objects with areas of different reflectance,” Procedings SPIE 4275, 85-93 (2001); and Martin Fay et al., “Measuring High-Slope and Super-Smooth Optics with High-Dynamic-Range Coherence Scanning Interferometry,” Optical Fabrication and Testing (OFT) 2014, OSA Technical Digest (online). However, this approach is necessarily affected by detector saturation limitations at the higher intensity modes of operation and it is still limited in the degree to which fringes can be obtained at very high slopes, especially when dealing with smooth surfaces.
A similar fringe deficiency is experienced even at lower slopes when the sample surface is rough and the light is not reflected toward the objective in sufficient quantity to provide visible interference fringes. This problem is accentuated by the relatively small objective aperture necessarily required for interferometric profilometry. Therefore, the task of measuring rough and/or high-slope sections of samples otherwise suitable for scanning interferometric measurements has been problematic in the art.
When the sample surface includes areas that cannot be measured interferometrically in their entirety on account of their slope or roughness, other, often irradiance-based, techniques have been utilized to fill in the gaps. One such method, for example, is the one commonly referred to in the art by the term contrast, referring to the fact that the surface is measured by finding the position of maximum contrast in the sample image (which corresponds to the best focus) produced by imaging through a conventional objective. Because of the normally higher numerical apertures used with conventional objectives, contrast methods produce results even when light scattering results from high slopes and/or from high surface roughness. Side illumination is sometimes added to such conventional (non-interferometric) objectives to increase irradiance to the sample, especially for higher slopes of both smooth and rough surfaces. The downside is that such non-interferometric methods are less accurate and are not suitable for measuring smooth surfaces because the exact focal position is hard to establish due to the lack of detectable features in the visible image of the sample.
Co-owned patent application Ser. No. 14/293,662 and Ser. No. 14/294,038 describe the use of side illumination for the purpose of enhancing color images generated from surfaces having low reflectivity. However, continued developments in the technologies covered by these patent applications has resulted in the discovery of heretofore unrecognized, significant, additional advantages of side illumination if combined with scanning interferometers. As described below, these advantages go a long way in overcoming the deficiencies outlined above with respect to measuring high-slope and rough surfaces.